The demand for increased bandwidth optical communication systems has lead to use of increasingly sophisticated lasers for signal transmission via multiple separate, concurrent data streams in a single optical fiber. Each data stream is modulated onto the output beam of a corresponding transmitter laser operating at a specific channel wavelength, and the modulated outputs from the lasers are combined onto a single fiber for transmission in their respective channels. The International Telecommunications Union (ITU) presently requires channel separations of approximately 0.4 nanometers, or about 50 GHz. This channel separation allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers. Improvements in fiber technology together with the ever-increasing demand for greater bandwidth will likely result in smaller channel separation in the future, and require greater precision from laser transmission devices.
In order to maximize optical transmission power and maintain wavelength stability in telecommunication transmitter lasers, steps are taken during transmitter manufacturing, assembly, and operation to minimize optical losses associated with laser operation. Two methods are generally used to characterize optical loss; measuring the laser output power, and measuring the laser threshold current. For example, in an external cavity diode laser, an end mirror is used to direct or feed back light into the gain medium. If the end mirror tilts or is not properly adjusted, a loss in the amount of light fed back into the cavity occurs. Once the end mirror is adjusted to a loss minimum, the mirror may be fixed in place at the time of manufacture. Alternatively, the loss associated with the end mirror tilt can be monitored during operation and continually minimized. An example is the adjusting of the period of a Bragg grating which feeds back light into a gain section of a DFB (distributed feedback) laser.
Both optical power measurement and threshold measurement have drawbacks when used to identify loss minima. In measuring optical power, a loss minimum does not necessarily correspond to an output power maximum. Assuming that the loss is not perfectly distributed throughout the gain medium, for example, when the loss occurs outside the gain media, then the laser will shift its internal distribution of power to send more power toward the loss. An increase in the loss may cause the power, as measured by the output of the laser, to increase, stay the same, or decrease depending on particular circumstances. The equations which relate output power to cavity losses are often very difficult to solve, even qualitatively, making output power an unreliable indicator of the intracavity loss associated with the alignment of a particular loss element. The total power exiting the gain medium is a relatively reliable indicator of relative cavity loss, but this quantity is difficult to measure.
Measuring laser threshold current to determine loss minima also has many drawbacks. The advantage of measuring laser threshold current is that a minimum of laser threshold current corresponds to a minimum of cavity loss. Laser threshold current is normally determined by adjusting the current injected into a gain region to determine the current where laser threshold is first observed. An important drawback is that changing the injection current changes the optical path length or thickness of the gain medium because the temperature of the gain medium (and thus its dimensions) changes along with other effects. A change in optical path length changes the wavelength of laser operation, and when loss must be minimized in-cavity with other losses which are wavelength dependent, or when loss must be minimized at a specific frequency of operation, the laser threshold is difficult or impossible to measure. Furthermore, laser threshold current cannot be used to minimize losses at high power and high current or constant power or constant current because it is measured at a current where lasing action is first observed.
As increasingly sophisticated transmitter lasers are required to meet increased band width needs, improved systems and methods for loss evaluation will be required, together with the ability to correct for losses which arise after manufacturing and assembly and losses which are related to the operation of the laser.
The invention provides systems and methods for probing or evaluating optical loss characteristics associated with lasers utilizing semiconductor gain media by monitoring voltage across a laser gain region. The invention also provides a method of laser operation wherein intracavity losses are determined by monitoring voltage across a gain region, a method for adjusting intracavity loss elements during laser operation to optimize loss profiles associated with the various loss elements, and a method for wavelength stabilization and control in external cavity lasers. The invention utilizes the fact that optical feedback into the gain region from loss elements outside the gain region is accurately detectable in the voltage across the gain region during laser operation.
The invention, in one embodiment, is a method for controlling or operating a laser cavity comprising monitoring voltage across a gain region emitting a coherent beam along an optical path, and determining optical losses associated with the laser cavity according to the monitored voltage. The method may further comprise adjusting a loss characteristic of the laser cavity according to the monitored voltage across the gain medium. The adjustment of the loss characteristic may comprise adjusting the position or other property of a loss element positioned in the optical path of a laser cavity. The laser may be an external cavity laser, and the loss element may comprise, for example, the end mirror, or a tunable filter placed external to a semiconductor gain medium.
There may be multiple additional loss elements present in the optical path or otherwise associated with the external cavity, such as a grid generator, channel selector, collimating optics, polarizing optics and other optical components, and losses associated with each such element may be evaluated, and adjustment of each loss element may be carried out, according to the monitored voltage across the gain medium.
In certain embodiments, evaluation of loss characteristics and adjustment of the loss element may be carried out by introducing a frequency modulation or dither to the loss element that is detectable in the monitored voltage. An error signal indicative of the propagation characteristics of the frequency dither, and hence the loss characteristics associated with the loss element, is derived from the monitored voltage, and is used to adjust the loss element to control the laser cavity loss profile. Where multiple loss elements are present, a separate frequency dither may be introduced to each loss element to provide corresponding error signals indicative of loss characteristics associated with each loss element. The introduction of the frequency dither to each loss element, as well as the adjustment of each loss element, may be carried out sequentially. Alternatively, different, non-interfering frequency dithers may simultaneously be introduced to each of the loss elements such that the propagation characteristics of each frequency dither are detectable in the monitored voltage across the gain medium, and such that each loss element may be simultaneously adjusted to control the loss profile of the external cavity.
A dither may also be employed wherein a loss element is varied among two or more positions and the laser voltage measured at each position. The nominal operating point may then be set to the position with the better laser voltage. Multiple elements may be optimized in this way by sequencing the dithering of each element to occur at different times.
Multiple dither elements may be used in association with multiple degrees of positional freedom of the loss element such that each dither element produces a frequency dither capable of being detected in the monitored voltage. Errors signals derived from the frequency dithers are used by the control system to positionally adjust the multiple degrees of positional freedom of the loss element. Multiple dither elements may also be used in association with multiple loss elements to allow simultaneous or sequential evaluation of loss characteristics associated with each loss element associated with laser cavity, and corresponding adjustment of each loss element to control the loss profile of the laser cavity.
In another embodiment, the invention is an external cavity laser apparatus comprising a gain medium emitting a coherent beam along an optical path, an end mirror positioned in the optical path such that the end mirror and a rear facet of the gain medium define an external cavity, and a voltage sensor operatively coupled to the gain medium and configured to monitor voltage across the gain medium. The monitored voltage across the gain medium is indicative of optical losses associated with the external cavity, and may be used to control the external cavity loss profile. The external laser cavity may comprise a control system operatively coupled to the voltage sensor and to one or more loss elements in the optical path in the external cavity, with the control system configured to adjust the loss element(s) according to monitored voltage across the gain medium.